I like kosoto-gari, and it's one of the few throws I can sometimes get just by "going for it". So, I go for it a lot. Can anyone provide some good set-ups and/or follow-ups for kosoto attempts? It's pretty similar throw to Deashi Harai. Will the set and follow-ups for that work for kosoto too?

As follow ups:

From a failed ko soto gari you can go straight into tani otoshi or you can set up yoko tomoe nage with it. The second combo is a bit bizzare though, in essence, if you get lift on their leg but for whatever reason can not throw them, you plant your sweeping foot, rotate around it while they are still off balance with one foot in the air, and move into yoko tomoe nage with what was originally your standing foot in their stomach.

I like kosoto-gari, and it's one of the few throws I can sometimes get just by "going for it". So, I go for it a lot. Can anyone provide some good set-ups and/or follow-ups for kosoto attempts? It's pretty similar throw to Deashi Harai. Will the set and follow-ups for that work for kosoto too?

We drilled kosoto gari to tai otoshi a couple days ago. It seemed surprisingly effective. Assuming standard grip and right side kosoto, when you go in for it a natural defence is for them to step/hop to your left to get away from the reap. When they do that they've created the space for tai otoshi: plant your reaping foot, turn and step back with your left, shoot the right out low and tai otoshi.

I LOVE this throw and it is relatively under utilized so I usually surprise people with it. It also works with that annoying bent over BJJ posture and you can go right to knee on belly!

Sasae has been good to me - I throw people outright with it and it's a great entry into Uchimata for me. I've even been finding Ippon Seionage off it, which, for a reasonably tall guy with broad shoulders, isn't the easiest of throws.

Ko Soto combo - I've been working on a version of Uchimata lately, where you throw from quite far in front of Uke's front left corner, making contact quite low on Uke's leg with the lower part of your lifting leg. I guess it's a Ko Uchimata or something.
One of the ways into it that I came up with for getting to this was off a Ko Soto. Worked quite well.

Personally I think ren raku waza is a bit over rated. Before people freak out, you need to look at why you're doing it. The primary reason to use it is to move your opponent into a specific position to be able to do the main technique you are setting up. I find that sometimes people focus too much on using set up techniques to do this rather than using gripping and kuzushi which are more efficient and effective.

In higher level judo my experience is that combinations are definately used but not in so much of a choreographed way. They tend to happen spontaneously as your opponent reacts to your intial attack (which has the intent to throw). Obviously there will be some more typical reactions that happen fairly consistantly and on that basis you're going to start using that combination. Spontaneously you will come up with combinations that happen on the fly and often without really thinking about them. You will simply see or feel an opening for one of your primary throws and do it without having planned it as you entered into the initial technique.

Using grips and kuzushi is generally safer than using an attack to set up your opponent since you can't be countered as easily. The danger with using combinations is that it takes the emphasis off of using your gripping to set up your techniques. As well you get reliant on them. In my case I can't do yoko-tomo-nage without using ko-soto to set it up.

I guess my main point is not to get hung up on using combos. Let them happen spontaneously and drill them from there. Look at what situation you're trying to create and see if you can create it with simple gripping and pulling instead.